1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
A well-known anti-skid control apparatus for a vehicle braking system includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressure control valve device receiving control signals from a control unit that measures the skid condition of the wheel to control brake fluid pressure to the wheel cylinder. A hydraulic reservoir receives brake fluid discharged through the fluid pressure control valve device from the wheel cylinder when the device decreases brake fluid pressure to the wheel cylinder, a pressure fluid supply conduit connects the master cylinder with the fluid pressure control valve device, and a fluid pump returns the brake fluid from the hydraulic reservoir into the pressure fluid supply conduit.
When the fluid pressure control valve device is provided for each of the four wheels, and their fluid pressure is independently controlled, there is no problem of control. When the fluid pressure control valve device is provided for each of the front wheels, and for both rear wheels in common, there is also no problem of control. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower of the speeds of the rear wheels. However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large and very heavy. Since each fluid pressure control valve device is expensive, the system is expensive.
Suppose the brake fluid pressures of the front wheels are controlled by two fluid pressure control valve devices in the diagonal or X-type conduit system, and the brake fluid pressures of the rear wheels are controlled together with the front wheels. When the vehicle runs on the right and left sides a road on which are considerably different in frictional coefficient, there is the danger that the one rear wheel that is diagonally opposite to the one front wheel on the higher frictional coefficient side will become locked. In that case, steering of the vehicle becomes unstable. That is very dangerous.
Further, suppose that proportioning valves are provided for the rear wheels. The brake fluid pressures of the rear wheels increase in proportion to the input fluid pressures to the proportioning valves. There is danger of locking.
Accordingly, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small and light, and can minimize the danger of locking of the rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system which includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressure control valve device receiving control signals from a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder. A hydraulic reservoir which, when the brake fluid pressure to the wheel cylinder is decreased with control of the fluid pressure control valve device, stores the brake fluid discharged through the fluid pressure control valve device from the wheel cylinder. A pressure fluid supply conduit connects the master cylinder with the fluid pressure control valve device, and a fluid pump returns the brake fluid from said hydraulic reservoir into said pressure fluid supply conduit. The fluid pressure control valve devices are provided for a pair of front wheels, respectively, and a valve apparatus receiving fluid pressures of wheel cylinders of said front wheels is arranged between the pair of front wheels and a pair of rear wheels. When any one of the fluid pressure control valve devices starts to control the fluid pressure of at least the one of the rear wheels, thats at the same side as the one of the front wheels, the fluid pressure of the wheel cylinder which is lower than that of the other is controlled in accordance with the lower of the fluid pressures of the wheel cylinders of the front wheels by the valve apparatus.
In the above-described anti-skid control apparatus, the control signals of the control unit are formed by determining the skid conditions of the respective front wheels. On the assumption that the front and rear wheels are provided with tires of the same kind, the braking forces are so distributed to the wheels that the front wheels tend to lock sooner than the rear wheels, when the vehicle is rapidly braked on a road which is uniform in frictional coefficient.
However, when the above assumption is not fulfilled, as for example, when only the front wheels are provided with spiked tires or chains for running on snowy or icy road, and the rear wheels are provided with normal tires, the rear wheels tend to lock sooner than the front wheels. In the above anti-skid control apparatus, the brake fluid pressure is not controlled when the rear wheel locks. When the brake fluid pressure of the front wheel is higher than the limit locking pressure of the rear wheel, the the rear wheel is not unlocked and so steering stability cannot be maintained.
Even when the front and rear wheels are provided with tires of the same kind, the rear wheel may tend to lock sooner than the front wheel when the front wheel brakes fade, causing the limit lock pressure of the front wheel to become excessively high. This is particularly likely when the vehicle is rapidly braked on a high road. When a proportioning valve is used, the fluid pressure at the rear wheel is lower than that of the front wheel. However, the pressure at the rear wheel increases in proportion to the fluid pressure of the front wheel, and reaches the limit lock pressure. The above described problem occurs.
FIG. 1 shows the problem described above. FIG. 1A shows the changes of the wheel speeds during the time when the vehicle is braked. FIG. 1B shows the control signals of the control unit, and FIG. 1C shows the changes of the brake fluid pressures of the wheels.
When the front and rear wheels are provided with tires of the same kind, and they run on the road having a uniform frictional coefficient, the brake fluid pressures P and P' of the front and rear wheels change with time, as shown by the solid lines in FIG. 1C, when the brake pedal is depressed at time t0. The control unit generates a brake maintaining instruction at time t1. The fluid pressure control valve device comprises an inlet valve and an outlet valve. The control signals consist of signals EV and AV for the inlet and outlet valves respectively.
Although AV is still "0", EV becomes "1" at time t1. Thus, the brake fluid pressure P of the front wheel is held constant. The control unit generates a brake relieving instruction at time t2. Thus, EV is still "1", and AV change from "0" to "1". As shown in FIG. 1C, the brake fluid pressure P of the front wheel decreases. AV becomes "0" at time t3, while EV is still "1". Thus, the brake fluid pressure is held constant.
EV becomes "0" at time t4. The brake fluid pressure rises again. EV again becomes "1" at time t5. The brake fluid pressure is held constant. Hereafter, the brake pressure P increases in steps as described above. AV becomes "1" at time t6, while EV is "1". Accordingly, the brake fluid pressure P decreases.
In the manner described above, the brake fluid pressure P of the front wheel changes with time. The brake fluid pressure P' of the rear wheel is reduced by the proportioning valve and also changes with time in accordance with the brake pressure P of the front wheel. The proportioning valve causes a hysteresis phenomenon by which the brake fluid pressure P' of the rear wheel changes a little later than that P of the front wheel. However, this time lag is neglected in FIG. 1C.
Generally, a larger amount of brake fluid is required for a constant increase of brake fluid pressure in the lower pressure range under the influence of rigidity of the wheel cylinder in the rear wheel brake apparatus. Accordingly, the change range of the brake fluid pressure P' of the rear wheel is less than that of the front wheel, as shown in FIG. 1C.
The wheel speeds V and V' of the front and rear wheels change with time, as shown by the solid lines in FIG. 1A, in accordance with the above described changes of the brake fluid pressures. This provides effective anti-skid control, decreasing wheel speeds without locking the wheels.
However, when only the front wheels are provided with chains, or when the front brakes fade the limit lock pressure of the front wheel is increased. In this case, the brake fluid pressure P of the front wheel changes with time, as shown by dashed-lines in FIG. 1A. The pressure rises above the level of the brake fluid pressure shown by the solid line. On the other hand, the brake fluid pressure P' of the rear wheel changes beyond the rear limit lock pressure R, as shown by the dashed line. Hereafter, even when the brake fluid pressure P of the front wheel is decreased, the rear wheel cannot be prevented from locking, partly because the range of the change of the brake fluid pressure P' is less. The front wheel is prevented from locking, as shown by the dashed line in FIG. 1A but, the rear wheel is locked. The anti-skid control is not effective and steering stability is lost. This is very dangerous situations.